Capacitance responsive control circuits

ABSTRACT

Capacitance-responsive control circuits operable with a lowvoltage DC power source and operative to change the energization state of a relay in response to either an increase or decrease in detected capacitance. These circuits include the following features: positive hysteresis, the capability of measuring the capacitance to ground of the protected object which acts as the antenna, and the capability of providing an indication of the magnitude of leakage current in the event that said protected object is not properly insulated from ground.

United States Patent Atkins May 22, 1973 1 CAPACITANCE RESPONSIVE 3,492,542 1/1970 Atkins ..340/258 0 C NTROL CIRCUITS 0 OTHER PUBLICATIONS [75] Inventor: Carl E. Atkins, Montclair, NJ.

Millman & Taub, Pulse, Digital and Switching [73] Assignee: Wagner 1 Electric Corporation, waveforms MCGTaW-H'H 1965' 389-394" Newark, NJ.

Primary Exammer-John W, Caldwell Assistant ExaminerWi1liam M. Wannisky [22] Filed. Dec. 22, 1970 Atmmey Eyre, Mann & Lucas [21] Appl. No.: 100,586

[57] ABSTRACT 307/278 Capacitance-responsive control circuits operable with [51] Int. Cl. ..G08b 13/26 a low-voltage DC power source and operative to 1 Field of Search 258 change the energization state of a relay in response to 3 0/ either an increase or decrease in detected 307/64 capacitance. These circuits include the following features: positive hysteresis, the capability of measuring Referemes Cled the capacitance to ground of the protected object which acts as the antenna, and the capability of UNITED STATES PATENTS providing an indication of the magnitude of leakage 3,555,534 l/l97l Akers etal ..340/258 C current in the event that said protected object is not 3,588,866 6/1971 Schlafly ..340/276 properly insulated from ground, 3,267,288 8/1966 Maiden etal. 340/333 ux 3,521,184 7/1970 Bowker ..340/258 R 18 Claims, 5 Drawing Figures 3,418,649 12/1968 Williamson .....340/258 R 3,571,666 3/1971 McGuirk ..340/258 C 3,382,408 5/1968 Atkins ..340/258 R J00 Jflfl/I id a) .fififli j I00 no 1 7 A52 1 #677: ML? V 7 W520 L== )26 "7 M2 ll/J 1% lM Z00 A 1 /50 #6], //Z (J NWO ,,1, v J

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CAPACITANCE RESPONSIVE. CONTROL CIRCUITS BRIEF SUMMARY OF THE INVENTION The present invention relates to capacitanceresponsive control circuits operable with a low-voltage DC power source, which may be either an AC/DC conversation circuit or a standby battery, and responsive to either an increase or a decrease in detected capacitance to change the energization state of a relay. In a particularly advantageous application, these circuits serve to control a burglar alarm device connected. between a pair of the relay terminals.

The circuits which are the preferred embodiments of the invention disclosed herein include several novel features. First, the circuits incorporate a positive hysteresis feature, i.e., a predetermined increase or decrease in antenna capacitance to ground will cause a change in the energization state of the controlled relay, but a larger decrease or increase (respectively) in that capacitance will subsequently be required to cause the relay to return to its original energization state. Second, each circuit enables a direct reading of the capacitance to ground of the protected object which acts as the antenna by adding that capacitance to a known value of capacitance and then subtracting that objects capacitance from the known capacitance by adjustment of a range switch and a dial connected to a continuouslyvariable component of the known capacitance. Restoration of the normal circuit condition indicates that the known value of capacitance has been reached. Third, the circuits include the capability of providing an indication of the magnitude of leakage current flowing to ground from the object which acts as the antenna when that object is not properly insulated from ground. This indication is given by the rate at which a neon tube blinks, this rate being directly proportional to the magnitude of the leakage current. If leakage current is very high, the neon tube will be steady-on.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention may be had by reference to the drawings, of which:

FIG. 1 is a schematic diagram of the circuit which forms the first preferred-embodiment of the present invention;

FIG. 2 is a schematic diagram of the circuit which forms the second preferred embodiment of the present invention;

FIG. 3 illustrates the dial connected to the continuously-variable capacitance in each of FIGS. 1 and 2, the range switch connected to the discretelyvariable capacitance, and the calibrated dial face; and

FIGS. 4 and 5 are front and side elevations, respectively, of the apparatus employed for storing and dispensing the coaxial cable which connects the object to be protected to the control circuits.

DETAILED DESCRIPTION OF THE INVENTION Referring now specifically to the circuit shown in FIG. 1, an AC/DC conversion circuit 100 serves to convert standard alternating current power (110 volts, 60 hertz) to direct current power of approximately +l3 volts, this output being derived at the three-way junction of resistance 102, zener diode 118, and the base of transistor 122. The AC/DC conversion circuit 100 also provides a regulated DC trickle current derived at the low side of emitter-follower resistance 112 for charging the standby battery 200, which provides a nominal +12 volts DC power output. Both of these direct-current outputs are channellled through a function switch 300, which may be keyed to one of four settings: OFF, BAT- TERY, TEST and ARM.

In the OFF setting, the ganged armatures 304A, 304B, 304C and 304D are in contact with contacts 306A, 3068, 306C and 306D, respectively, as shown. Only two connections are thus established. The first connection is between the trickle current output of AC/DC conversion circuit and the standby battery 200 through contact 3063 and armature 3048 to maintain the battery 200 in a charged condition. Since diode 119 has a slightly higher voltage at its cathode as compared to its anode, no trickle current will flow therethrough. The second connection is between terminal 1012 of relay 1000 and one side of the tamper switch 002 through armature 304D and contact 306D. Thus, if the normally-open anti-tamper switch 002 is held closed by the closed door of the cabinet enclosing the circuit shown in FIG. 1, an alarm device connected between the terminals 1012 and 1010 of relay 1000 and normally energized by an independent power source will be de-energized if the cabinet door is opened.

In the BATTERY setting, the ganged armatures 304A, 304B, 304C and 304D are in contact with contacts 308A, 3088, 308C and 308D, respectively. A condition check of the standby battery 200 is provided by the ammeter reading of the current flowing from the '+l2 volts DC output terminal of the battery 200 through armature 304B, contact 3088, ammeter 314, armature 304C, contact 308C and resistor 302 to true ground (e.g., a waterpipe). Because of the slightly higher voltage at the cathode of diode 119 as compared to its anode, no trickle current will be diverted there through to cause an erroneous readout by ammeter 314. Tamper switch 002 is effectively removed from the circuit by the direct connection of relay terminals 1010 and 1012 via armature 304D and contact 308D, which prevents actuation of 'a normally-energized alarm device connected between terminals 1010 and 1012 in the event that the condition of the battery 200 has deteriorated to the point that the control circuit cannot maintain relay 1000 in an energized state.

In the TEST setting, the ganged annatures 304A, 304B, 304C and 304D are in contact with contacts 310A, 3108, 310C and 310D, respectively. Thus, the balance of the control circuit is now energized by the +13 volts DC power output of the AC/DC conversion circuit 100 derived at the cathode of diode 118 and channelled through armature 304A and contact 310A, ammeter 314, armature 304C and contact 310C. A condition check of AC/DC conversion circuit 100 is provided by the reading of ammeter 314. Also, terminals 1010 and 1012 of relay 1000 are directly connected through contact 310D and armature 304D, thus effectively removing the tamper switch 002 from the control circuit and preventing a normally-energized alarm device connected between relay terminals 1010 and 1012 from being actuated by de-energization of relay 1000.

Thus, in the TEST mode, adjustment and testing of the control circuit may be performed without actuating the alarm device. Specifically, with the collective capacitance 1100 set at its known maximum value, the oscillator 500 may be adjusted to produce nearly-null output pulses by means of variable resistor 512. Then, the object to be protected may be connected to collective capacitance 1100 by means of terminal 1122, which will cause de-energization of relay 1000 by virtue of the addition of the increment of capacitance formed by that object. The collective capacitance 1100 may then be adjusted by varying the connection of the armature 1106 of range switch 1105 to obtain a rough adjustment and by varying the continuously-variable capacitance 1116 to obtain fine grain adjustment, until the relay 1000 is again energized. At this point, the approximate value of the capacitance of the object to be protected may be read directly from the dial shown in FIG. 3. Because the control circuit has upper and lower thresholds determined by the maximum predetermined positive and negative incremental variations in capacitance which will cause de-energization of the relay 1000, and because these thresholds may be fairly close to one another, adjustment of the circuit is facilitated by the calibration circuity which is operative to make the control circuit unresponsive to the negative minimum predetermined incremental change in capacitance detected by the object to be protected, which serves as the antenna for the control circuit. This calibration circuitry comprises a pair of normally-open ganged switches 518 and 624, the former being connected in series with capacitance 514 between the object to be protected (via the inner conductor of coaxial cable 1118) and true ground, and the latter being connected in series with capacitance 618 across the output terminals of the first stage of amplifier 600. Thus, when the ganged switches 518 and 624 are closed, the control circuit will have only one threshold, i.e., the upper threshold. In carrying out these adjustments, the audible click produced by the de-energization and energization of relay 1000 provides an aural indication when a threshold has been reached.

In the ARM setting, the ganged armatures 304A, 3048, 304C and 304D are in contact with contacts 312A, 312B, 312C and 312D, respectively. The tamper switch 002 is again effectively removed from the circuit, but the relay terminals 1010 and 1012 are no longer connected through armature 304D because contact 312D is floating. Consequently, if relay 1000 is deenergized, its armature 1004 will move from contact 1006 to contact 1008, thus causing de-energization of a normally-energized alarm device connected across relay terminals 1010 and 1012. v The +13 volts DC power output of AC/DC conversion circuit 100 is applied to the balance of the control circuit via armature 304A and contact 312A, through ammeter 314, and through armature 304C and 312C. Standby battery 200 is again connected to the trickle current output of AC/DC conversion circuit 100 via armature 3043 and contact 3128.

The DC/DC convertor 400 provides two outputs. The first output of approximately +175 volts DC is derived at the cathode of diode 426 and energizes the low-frequency relaxation oscillator 500. The second output of approximately 5 to -l0 volts DC is derived at the anode of diode 428 and is applied as a bias voltage to the gate electrode of switching circuit 700 (i.e., to the base of transistor 716) to maintain that circuit normally non-conductive. The output pulses of oscillator 500 may be initially adjusted in polarity and magnitude by means of variable resistor 512. In the present embodiment, the pulses are adjusted to be nearly null.

Then, when the antenna (the protected object) detects an increase in capacitance, the output pulses will become negative, and when the antenna detects a decrease in capacitance, the output pulses will become positive.

The output pulses of oscillator 500 are fed to the two stage amplifier 600 via a capacitive coupling network comprising capacitances 612 and 614. A significant feature of the present invention resides in the interconnection of the oscillator 500, which has a relatively high output impedance, to the two-stage amplifier 600, which has a relatively low input impedance, by a relatively small bridging capacitor 614 interconnecting the output and input circuits. Optionally, a second capacitor 612 may be connected across the output terminals of oscillator 500 to form a capacitive coupling network. This combination provides a turn-around phenomenon, i.e., whether the output pulses from the oscillator 500 are positive-going or negative-going, a positivegoing input pulse will be provided to the amplifier 600. When the output of the oscillator 500 comprises negative-going pulses, each of these pulses will have a positive-going overshoot which will be fed as positive input pulses to the input of the amplifier 600.

The amplified positive output pulses of amplifier 600 are in turn fed to the gate of the normally nonconductive DC switching circuit 700. This switching circuit maintains the emitter-follower amplifier 800 normally non-conductive, and the steady positive output of amplifier 800 maintains the first and second transistors '918 and 920, respectively, of the modified Schmitt trigger circuit 900 normally non-conductive and conductive, respectively. Thus, winding 1002 of the load-controlling relay 1000 is normally energized, and armature 1004 normally closes the path between terminals 1010 and 1012 and opens the path between terminals 1012 and 1014.

The oscillator 500 includes collective capacitance 1100, increments of which are' formed by a multiposition range switch 1105, a variable capacitance 1116, a coaxial cable 1118 enabling electrical connection to the protected object which acts as antenna by means of terminal 1122, and the capacitance to ground of said object. The maximum value of this collective capacitance 1100 without an object connected to the coaxial cable is known. When connections is madeto an object, both the range switch 1105 and the variable capacitance 1116 are adjusted to subtract the increment of capacitanceadded by the protected object. A calibrated dial face (FIG. 3) operatively associated with dials connected to the range switch and the variable capacitance gives a direct reading of the value of the capacitance to ground of the protected object. The case in which the control circuit is enclosed is preferably connected into collective capacitance 1 by terminal 1120 through capacitance 1104, which reduces shock hazard, and parallel-connected resistance 1102, which provides a DC current path to enable the leakage test circuitry (502, 520, 522) to provide an indication of leakage current from the case as well as from the protected object.

When the modified Schmitt trigger circuit 900 is actuated, i.e., when the conductivity states of transistors 918 and 920 are reversed, energization current will no longer flow to the winding 1002 of relay 1000, but current will now flow through resistor 910 to provide a positive voltage to the gate electrode of switching circuit 700 through resistor 706. This voltage is of opposite polarity with respect to the constant bias voltage provided by the DC/DC convertor 400, and has the effect of making the net bias on the switching circuit 700 less negative. Thus, when the output relay 1000 has been tie-energized, as a result of either an increase or a decrease in capacitance to ground of the object acting as antenna, a larger decrease or increase (respectively) in that capacitance will be required to reduce the magnitude of the positive pulses provided by amplifier 600 in order to enable re-energization of the relay 1000. This is the earlier-mentioned positive hysteresis feature.

In the preferred embodiment of the invention shown in FIG. 1, the values of the various circuit elements are as follows:

Resistances 102 470 ohms 104 2.2K ohms Capacitances 114 2S0 microfarads 410 100 microfarads 106 1K ohms 412 0.33 microfarads 108 K ohms 414 4.7 nanofarads 110 10K ohms 416 3.3 nanofarads 112 240 ohms 418 0.22 microfarads 302 680 ohms 420 47 nanofarads 402 47K ohms 422 0.1 microfarad 404 33K ohms 424 4.7 nanofarads 406 100 ohms 514 0.1 nanofarad 408 100K ohms 502 2.2 megohms 504 4.7 megohms 506 1K ohms 516 0.1 microfarad 612 0.1 microfarad 614 0.01 microfarad 616 22 nanofarads 508 22K ohms 510 30K ohms 512 1K ohms (maximum) 602 l megohm 604 5.1K ohms 606 100 ohms 608 330K ohms 610 10K ohms 702 470K ohms 704 100K ohms 706 470K olrms 708 3Ehms 802 100K ohms 804 470K ohms 902 47K ohms 904 47 ohms 906 68K ohms 908 1.8K ohms 910 3.3K ohms 912 470 ohms 1102 '22 megohms Transistors 1 122 MJE521 124 M18521 126 M15521, 12a M15521 v 430 2N3567' I 620 2mm 622 2N3567 714 2N4248 716 2N3567 s06 2N4248 618 47 nanofarads 710 47 nanofarads 712 l5 nanofarads 914 0.5 nanofarad 916 80 microfarads 1104 0.2 nanofarad 1108 1.3 nanofarads 1110 0.9 nanofarad 1112 0.455 nanofarad 1116 90-5 80 picofagds 1118-41.5 feet of Alpha 1702 Coaxial Cable Diodes 120 series-connected base-emitter junctions of two 2N5138 transistors 426 1N5059 428 1N5059 Transformers 130 Primary: 110 VAC,60

hertz SecondaryzlZ VAC,60 hertz 432 .LW. Miller Coil Co.

Conversion Transformer 186267 Neon Tubes 522 Si nalite T2-27-1W250 524- ignalite T2-27-1W250 Referring now specifically to the circuit shown in FIG. 2, an AC/DC conversion circuit 100 serves to con- 60 diode 154 which is poled to block load current from the battery 200. The function switch 300 has three mode settings, OFF, TEST and ARM.

When set in the OFF position, the function switch 300 effectively removes ammeter 328 from the control circuit by opening the current path therethrough.

When set in either the TEST or ARM position, the ammeter 328 will indicate the magnitude of the current being supplied by the AC/DC conversion circuit to the balance of the control circuit. Although the +13 volt DC output power supplied by circuit 100 at the emitter of transistor 158 is fed directly to the balance of the control circuit, i.e., without being channelled through the function switch 300, a very small portion of the output current will pass through resistor 332, armature 320A, contacts 324A and 326A, and ammeter 328 to true ground. If AC/DC conversion circuit 100 is disconnected from the source of standard AC power, or if there is a failure of either the conversion circuit or the source of AC power, the output current of standby battery 200 is fed through diode 154 to the balance of the control circuit, again with a very small current portion flowing through resistor 332, armature 320A, interconnected contacts 324A and 326A, and ammeter 328 to true ground. Thus, regardless of which source of power the circuit is drawing upon, an indication will be given of the condition of that source of power when the function switch 300 is in either the TEST or ARM position. Control circuit testing and adjustment is carried out in the TEST mode by the procedure described in connection with the embodiment of FIG. 1. The calibration circuit which serves to render the control circuit unresponsive to an increase in detected capacitance comprises the normally-open switch 776 connected between the first input channel of OR gate 800 and true ground. The leakage test circuit comprises resistor 524, normally-open switch 542, and neon tube 540, all connected in series between the high-voltage output of DC/DC convertor 400 and the object to be protected via the internal conductor of coaxial cable 1138, noise-inhibiting inductance 1144, and terminal 1142. The cabinet in which the control circuit is enclosed (not shown) is preferably connected into the collective capacitance 1100 by terminal 1140 through noise-inhibiting inductance 1124.

Two DC voltages are generated by DC/DC convertor 400. The first DC voltage is of a high level volts) and is applied to the low-frequency relaxation oscillator 500. The second DC output is of relativelylow level (-5 to -10 volts) and is applied to the switching circuits comprising transistor pairs 762, 764 and 768, 770 in the signal-generating circuit 700. The output pulses of oscillator 500 are adjusted by means of variable resistance 532 so that, when amplified by the two-stage amplifier 600 and fed to the first switching circuit 762, 764, the negative tum-off bias applied through resistor 718 is overcome to maintain the switching circuit 762, 764 normally conductive with the application of each amplified pulse. Thus, the control circuit is less sensitive to noise, which is capable of turning on switching circuit 762, 764. A sawtooth wave will appear at the emitter of transistor 762 and will provide a low average DC input to the first input channel of OR gate 800 via resistor 724. The oscillator 500 is adjusted so that the input pulses appearing at the base of transistor 764 normally will have virtually no overshoot or undershoot on the trailing edges of the pulses. Under these conditions,

the detection circuit comprising capacitance 754, resistance 728, diode 774 and capacitance 756 will provide no input to the normally conductive transistor 766, the constant low-voltage output of which will have no effect on the normally non-conductive switching circuit 768, 770. Under these conditions, transistor 772 will be maintained normally conductive, thus resulting in a low-level positive output voltage at the collector thereof, which is fed into the second input channel of OR gate 800. Thus, under normal conditions, neither the first intermediate signal derived at the anode of switching circuit 762, 764 nor the second intermediate signal derived at the collector of transistor 772 will be sufficient to cause OR circuit 800 to generate a positive output, i.e., transistor 812 is maintained normally nonconductive. Thus, the normal condition of the modified Schmitt trigger circuit 900 is as follows:

transistor 936 is maintained non-conductive and transistor 938 is maintained normally conductive to close a current path from the source of power through winding 1002 of relay 1000, through the collector-emitter junction of transistor 938, and through current-limiting resistor 930 to ground. Resistor 932 and capacitance 934 are connected in series across the winding 1002 to provide a discharge path for the current induced in the winding when the normally-energized relay is deenergized.

If the circuit detects a decrease in the normal level of capacitance formed by the protected object and by the variable collective capacitance 1100, which decrease could be caused by breaking the connection between that object and coaxial cable 1138, the output pulses of oscillator 500 will increase in magnitude and, when amplified by amplifier 600, will appear at the base of transistor 764 with a negative overshoot on the trailing edge of each pulse. Thus, the first switching circuit 762, 764 will remain in its normal condition, i.e., intermittently conductive. The negative overshoot of the input pulse will be detected by the detection circuit connected between the input to the switching circuit 762, 764 and the base of transistor 766, thus providing a pulsating negative voltage at the base of transistor 766. Consequently, the normally-conductive transistor 766 will be pulsed less conductive, thus providing positive input pulses to the switching circuit 768, 770 to overcome the negative bias thereon. Consequently, the switching circuit 768, 770 is rendered conductive, thus lowering the level of the positive input into the base of transistor 772, which in'turn becomes non-conductive.

Consequently, a high positive input is provided to the signal will be required to render the switching circuit 768, 770 non-conductive again.

If the circuit detects an increase in the normal level of capacitance formed by the protected object and by the variable collective capacitance 1100, which increase could be formed by a person approaching that object, the output pulses of oscillator 500 will decrease in magnitude, and even after amplification by amplifier 600, will be too small when they appear at the base of transistor 764 to maintain the switching circuit 762, 764 in its normally-conductive condition. Consequently, the first switching circuit 762, 764 will become non-conductive. A high positive voltage will thus be developed at the emitter of transistor 762, and will be applied to the first input channel of OR gate 800 via resistance 724. The OR gate 800 will thereupon provide a positive input signal to the trigger circuit 900 in the manner described above. Also, the trigger circuit 900 and relay 1000 will perform as described above, with the exception that a hysteresis signal is not fed back to the gate electrode of switching circuit 762, 764. Of course, provision may be made for hysteresis signal feedback to this first switching circuit by the same sort of connection as employed with the second switching circuit.

In the preferred embodiment of the invention shown in FIG. 2, the values of the various circuit elements are as follows:

Resistances Ca acitances 136 10 ohms 14 .1 microfarad 138 1K ohms 144 160 microfarads 10 ohms 146 160 microfarads 332 220K ohms"(depends on) 442 100 microfarads 334 100K ohms"(ammeter 328) 434 47K ohms 436 33K ohms 438 470 ohms 440 3.3K ohms 524 1.5 megohms 526 4.7 megohms 528 22K ohms 530 12K ohms 532 10K ohms (maximum) 626 6.8K ohms 628 l8 megohms 630 220 ohms 632 1K ohms 634 47 ohms 636 330K ohms 638 10K ohms 718 330K ohms 720 470K ohms 722 270K ohms 724 100K ohms 726 39 ohms 728 470K ohms 730 10K ohms 732 330K ohms 734 5.1K ohms 736 220K ohms 738 l megohm 740 220K ohms 742 470K ohms 744 39 ohms 746 470K ohms 748 68K ohms 808 l megohm 810 47K ohms 922 47K ohms 924 3.3K ohms 926 l.5Kohms 444 .33 micorfarad 446 4.7 nanofarads 448 4.7 nanofarads 450 47 nanofarads 452 68 nanofarads 454 .1 microfarad 456 4.7 nanofarads 534 .l microfarad 640 .01 microfarad 642 47 nanofarads 644 l5 microfarads 646 .01 microfarad 648 .47 microfarad 750 .02 microfarad 752 .15 microfarads 754 22 nanofarads 756 .01 microfarad 758 .01 microfarad 760 .33 microfarad 933 .5 nanofarads 934 32 microfarads 1016 .l microfarad 1018 .l microfarad 1128 1.3nanofarads 1130 .9 nanofarads 1132 .455 nanofarads 1136 90-580 1124 .56 millihenry 1144 .56 millihenery Diodesv 1502N5l38(2), e-b jets. in series picofarads 1138 41.5 feet of Alpha 928 47K ohms 152 --1N5059 930 220 ohms 154 1N5059 932 470 ohms 460 1N5059 462 1N5059 Transistors 774 1N5059 158 2N5135 814 1NS059 160 MJRSZI 816 1N5059 464 2N3567 650 2N3567 Transformers 652 2N3567 162 Primary: 110 vac., 60 hertz 762 2N4248 Secondaryz12 vac., 60 hertz 764 2N3567 766 2N3567 466 .I.W. Miller Coil Co.

Conversion 768 2N4248 Transfonner 1136267 770 2N3567 772 2N3567 Neon Tubes 812 2N3567 538 Signalite T2-27-1W250 936 2N3567 540 Signalite T2-27-1W250 938 2N3567 Referring now specifically to FIG. 3, this figure illustrates the means for taking a direct reading of the capacitance to ground of the protected object which acts as the antenna for the capacitance-responsive control circuit. There are provided four ranges, the first extending from to 480 picofarads, the second extending from 380 to 370 picofarads, the third extending from 850 to 1325 picofarads and the fourth extending from 1300 to 1800 picofarads. The selection of a particular range of values is made by setting the dial 1106A, which is mechanically connected to armature 1106 shown in FIG. 1. Then, a finer adjustment of the net capacitance 1100 is effected by adjustment of dial 1116A, which indicates the value of capacitance to ground of the protected object of the selected range.

Referring now specifically to FIGS. 4 and 5, these illustrate the apparatus which is employed for storing and dispensing the coaxial cable 1118 which connects the protected object to the control circuit. The inner conductor of the coaxial cable is connected to the protected object by the connector 1122. In the control circuit, the inner conductor of coaxial cable 1118 is connected to the range switch 1105 at the high side of the capacitances 1108, 1110 and 1112, and also to the high side of the continuously variable capacitance 1116. Preferably, the inner conductor of coaxial cable 1118 is also connected to the case in which the control circuit is enclosed via the network comprising parallel resistance 1102 and capacitance 1104 and the connector 1120. This inner conductor is also connected to lowfrequency relaxation oscillator 500 at the high side of capacitance 516 therein. The outer conductor of coaxial cable 11 18 is connected to the low-frequency relaxation oscillator 500 at the high side of resistance 508.

As may be seen from the illustrations of FIGS. 4 and 5, the coaxial cable 1118 is stored on a spool 1124 which is mounted on a shaft 1126 containing a slot 1128. In the slot 1128, a key 1130 is slidably positioned. The key 1130 is in engagement with a suitablysized slot in the spool 1124. Thus, the spool 1124 may not be rotated about shaft 1126, which extends from the rear wall of the cabinet enclosing the control circuit. However, after opening the cabinet door, the spool 1124 may be moved on a shaft 1126 away from the cabinet wall and out of the narrow confines of the cabinet to facilitate the unwinding of a length of coaxial cable 1128 from the spool. Thus, the required length of coaxial cable 1118 may readily be freed from the spool and extended through a slot on the side of the cabinet (not shown). After the required length of coaxial cable has been unwound, the spool 1124 may be moved toward the rear wall of the cabinet along the shaft 1126, and the door to the cabinet may then be closed. Although this improvement has been described in relation to the circuit shown in FIG. 1, it is readily apparent that it may be employed with equal facility and benefit in the circuit shown in FIG. 2.

The advantages of the present invention, as well as certain changes and modifications of the disclosed embodiments thereof, will be readily apparent to those skilled in the art. It is the applicants intention to cover all those changes and modifications which could be made to the embodiments of the invention herein chosen for the purposes of the disclosure without departing from the spirit and scope of the invention.

What is claimed is:

1. A capacitance-responsive control circuit comprisl. power circuit means operative to supply direct current power at a plurality of voltage levels to the balance of the control circuit;

2. signal circuit means operative to generate a control signal of predetermined polarity in response to both minimum predetermined positive and negative incremental changes in detected capacitance from a predetermined reference level; and

3. load-controlling circuit means operative in response to said control signal to change the energization state of a load and to provide a hysteresis signal to said signal circuit means, which is subse quently operative to cease generation of said control signal only in response to an incremental change in detected capacitance which is greater than and of opposite sign with respect to said minimum predetermined incremental change.

2. The control circuit according to claim 1 wherein said signal circuit means comprises:

1. low-frequency relaxation oscillator circuit means operative to generate variable output pulses;

2. first amplifier circuit means operative in cooperation with said oscillator circuit means to generate amplified pulses of a first polarity in response to oscillator circuit means output pulses of either a first or a second polarity;

. 3. first switching circuit means biased normally nonconductive by said low-voltage direct current out put of said conversion circuit means and controlled by the output of said amplifier circuit means; and

4. second amplifier circuit means controlled by the output of said first switching circuit means and maintained normally non-conductive thereby.

3. The control circuit according to claim 2 wherein said first amplifier circuit means includes a capacitive input circuit connected to the output of said lowfrequency oscillator circuit means to cause said variable output pulses of said second polarity to overshoot and thereby develop pulses of said first polarity.

4. The control circuit according to claim 2 wherein said low-frequency relaxation oscillator circuit means comprises a collective capacitance formed by:

1. a multi-position capacitance range switch;

2. a continuously-variable capacitance;

3. a predetennined length of coaxial cable having an inner conductor and an outer conductor; and

4. the capacitance to ground of the object which acts as the antenna.

5. The control circuit according to claim 4 wherein said collective capacitance, excluding the capacitance to ground of the object which acts as the antenna, is of a known maximum value.

6. The control circuit according to claim 4 further including leakage test circuit means comprising a resistance, a normally-open switch, and a neon tube connected in series between said high-voltage direct circuit output of said power circuit means and the object which acts as the antenna via the inner conductor of said coaxial cable.

7. The control circuit according to claim 1 further comprising calibration circuit means operative to prevent said signal circuit means from generating said control signal in response to a minimum predetermined incremental change in detected capacitance of one polarlty.

8. The capacitance control circuit according to claim 1 wherein said signal circuit means comprises:

1. low-frequency relaxation oscillator circuit means operative to generate variable output pulses;

2. amplifier circuit means operative to amplify said variable output pulses;

3. first signal generating means operative to provide a first intermediate signal;

4. second signal generating means operative to provide a second intermediate signal;

5. detection circuit means interconnecting the input of said first signal generating means and the input of said second signal generating means, and operative to develop a controlling input signal for said second signal generating means in response to negative overshoot of the input signal of said first signal generating means; and

6. OR gate means operative to provide said control signal in response to either said first or said second intermediate signal.

9. The control circuit according to claim 8 wherein said low-frequency relaxation oscillator circuit means comprises a collective capacitance formed by:

l. a multi-position capacitance range switch;

2. a continuously-variable capacitance;

3. a predetermined length of coaxial cable having an inner conductor and an outer conductor; and

4. the capacitance to ground of the object which acts as the antenna.

10. The control circuit according to claim 9 wherein said collective capacitance, excluding the capacitance to ground of the object which acts as the antenna, is of a known maximum value.

11. The control circuit according to claim 9 further including leakage test circuit means comprising a resistance, a normally-open switch, and a neon tube connected in series between said high-voltage direct circuit output of said power circuit means and the object which acts as the antenna via the inner conductor of said coaxial cable.

12. The control circuit according to claim 1 wherein said load-controlling circuit means comprises trigger circuit means including first and second transistors normally maintained in non-conductive and conductive states, respectively, and operative in response to said control signal from said signal circuit means to reverse the conductivity states of said first and second transistors, and further operative upon said change of conductivity states to provide said hysteresis signal to said signal circuit means.

13. The control circuit according to claim 12 wherein said load-controlling circuit means further comprises a relay maintained in a normally energized condition by said trigger circuit means, said relay being de-energized by the reversal of the conductivity states of said transistors.

14. The control circuit according to claim 12 wherein said trigger circuit means is further operative in response to said control signal to divert the load current through an ammeter to provide a reading of the load current.

15. The control circuit according to claim 5 further comprising indicia associated with said multi-position capacitance range switch and said continuouslyvariable capacitance to enable direct reading of the capacitance of the object which acts as the antenna when said continuously-variable capacitance is connected to said object and said collective capacitance has been decreased from its known maximum value to a value which when summed with the capacitance of said object is equal to said known maximum value.

16. The control circuit according to claim 7 wherein said calibration circuit means comprises first and second ganged, normally open switches in said signal circuit means operative to prevent the generation of said control signal in response to a minimum predetermined negative incremental change in detected capacitance.

17. The control circuit according to claim 10 further comprising indicia associated with said multi-position capacitance range switch and said continuouslyvariable capacitance to enable direct reading of the capacitance of the object which acts as the antenna when said continuously-variable capacitance is connected to said object and said collective capacitance has been decreased from its known maximum value to a value which when summed with the capacitance of said object is equal to said known maximum value.

18. The control circuit according to claim 13 further comprising function switch means operative to enable selection of one of a plurality of predetermined modes of operation of said control circuit.

* t t t 

1. A capacitance-responsive control circuit comprising:
 1. power circuit means operative to supply direct current power at a plurality of voltage levels to the balance of the control circuit;
 2. signal circuit means operative to generate a control signal of predetermined polarity in response to both minimum predetermined positive and negative incremental changes in detected capacitance from a predetermined reference level; and
 3. load-controlling circuit means operative in response to said control signal to change the energization state of a load and to provide a hysteresis signal to said signal circuit means, which is subsequently operative to cease generation of said control signal only in response to an incremental change in detected capaCitance which is greater than and of opposite sign with respect to said minimum predetermined incremental change.
 2. amplifier circuit means operative to amplify said variable output pulses;
 2. signal circuit means operative to generate a control signal of predetermined polarity in response to both minimum predetermined positive and negative incremental changes in detected capacitance from a predetermined reference level; and
 2. The control circuit according to claim 1 wherein said signal circuit means comprises:
 2. first amplifier circuit means operative in co-operation with said oscillator circuit means to generate amplified pulses of a first polarity in response to oscillator circuit means output pulses of either a first or a second polarity;
 2. a continuously-variable capacitance;
 2. a continuously-variable capacitance;
 3. The control circuit according to claim 2 wherein said first amplifier circuit means includes a capacitive input circuit connected to the output of said low-frequency oscillator circuit means to cause said variable output pulses of said second polarity to overshoot and thereby develop pulses of said first polarity.
 3. a predetermined length of coaxial cable having an inner conductor and an outer conductor; and
 3. a predetermined length of coaxial cable having an inner conductor and an outer conductor; and
 3. load-controlling circuit means operative in response to said control signal to change the energization state of a load and to provide a hysteresis signal to said signal circuit means, which is subsequently operative to cease generation of said control signal only in response to an incremental change in detected capaCitance which is greater than and of opposite sign with respect to said minimum predetermined incremental change.
 3. first switching circuit means biased normally non-conductive by said low-voltage direct current output of said conversion circuit means and controlled by the output of said amplifier circuit means; and
 3. first signal generating means operative to provide a first intermediate signal;
 4. the capacitance to ground of the object which acts as the antenna.
 4. the capacitance to ground of the object which acts as the antenna.
 4. second amplifier circuit means controlled by the output of said first switching circuit means and maintained normally non-conductive thereby.
 4. The control circuit according to claim 2 wherein said low-frequency relaxation oscillator circuit means comprises a collective capacitance formed by:
 4. second signal generating means operative to provide a second intermediate signal;
 5. detection circuit means interconnecting the input of said first signal generating means and the input of said second signal generating means, and operative to develop a controlling input signal for said second signal generating means in response to negative overshoot of the input signal of said first signal generating means; and
 5. The control circuit according to claim 4 wherein said collective capacitance, excluding the capacitance to ground of the object which acts as the antenna, is of a known maximum value.
 6. The control circuit according to claim 4 further including leakage test circuit means comprising a resistance, a normally-open switch, and a neon tube connected in series between said high-voltage direct circuit output of said power circuit means and the object which acts as the antenna via the inner conductor of said coaxial cable.
 6. OR gate means operative to provide said control signal in response to either said first or said second intermediate signal.
 7. The control circuit according to claim 1 further comprising calibration circuit means operative to prevent said signal circuit means from generating said control signal in response to a minimum predetermined incremental change in detected capacitance of one polarity.
 8. The capacitance control circuit according to claim 1 wherein said signal circuit means comprises:
 9. The control circuit according to claim 8 wherein said low-frequency relaxation oscillator circuit means comprises a collective capacitance formed by:
 10. The control circuit according to claim 9 wherein said collective capacitance, excluding the capaCitance to ground of the object which acts as the antenna, is of a known maximum value.
 11. The control circuit according to claim 9 further including leakage test circuit means comprising a resistance, a normally-open switch, and a neon tube connected in series between said high-voltage direct circuit output of said power circuit means and the object which acts as the antenna via the inner conductor of said coaxial cable.
 12. The control circuit according to claim 1 wherein said load-controlling circuit means comprises trigger circuit means including first and second transistors normally maintained in non-conductive and conductive states, respectively, and operative in response to said control signal from said signal circuit means to reverse the conductivity states of said first and second transistors, and further operative upon said change of conductivity states to provide said hysteresis signal to said signal circuit means.
 13. The control circuit according to claim 12 wherein said load-controlling circuit means further comprises a relay maintained in a normally energized condition by said trigger circuit means, said relay being de-energized by the reversal of the conductivity states of said transistors.
 14. The control circuit according to claim 12 wherein said trigger circuit means is further operative in response to said control signal to divert the load current through an ammeter to provide a reading of the load current.
 15. The control circuit according to claim 5 further comprising indicia associated with said multi-position capacitance range switch and said continuously-variable capacitance to enable direct reading of the capacitance of the object which acts as the antenna when said continuously-variable capacitance is connected to said object and said collective capacitance has been decreased from its known maximum value to a value which when summed with the capacitance of said object is equal to said known maximum value.
 16. The control circuit according to claim 7 wherein said calibration circuit means comprises first and second ganged, normally open switches in said signal circuit means operative to prevent the generation of said control signal in response to a minimum predetermined negative incremental change in detected capacitance.
 17. The control circuit according to claim 10 further comprising indicia associated with said multi-position capacitance range switch and said continuously-variable capacitance to enable direct reading of the capacitance of the object which acts as the antenna when said continuously-variable capacitance is connected to said object and said collective capacitance has been decreased from its known maximum value to a value which when summed with the capacitance of said object is equal to said known maximum value.
 18. The control circuit according to claim 13 further comprising function switch means operative to enable selection of one of a plurality of predetermined modes of operation of said control circuit. 